The Dow Jones Transportation Average — a 20-member index of airlines, railroads and trucking companies — declined 5.9% this week, the largest weekly fall since November 2011. The index is at its lowest level since June and is down 2.2% this year.

Meanwhile, the more commonly followed Dow Jones Industrial Average has inched up this week and currently hovers around its highest level since December 2007. Including Friday’s drop, the Dow has had only four down sessions in September, which is historically the worst month of the year for stocks. The blue-chip index is up about 11% this year, led higher by Bank of America, Home Depot and Walt Disney, while Hewlett-Packard, McDonald’s and Boeing have lagged.

“This is a major divergence that should not be ignored,” Peter Boockvar, a managing director at Miller Tabak & Co., said in a chat with MarketBeat. “It tells me the risks of being in the market at these levels are growing.”

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